1. Field of the Invention
This invention relates to an image forming apparatus, and more specifically, to an image forming apparatus which corrects the slant of a document image.
2. Description of the Prior Art
Image forming apparatuses which are based on the principle of electronic photography are required to efficiently process a large number of documents. They therefore comprise an automatic document feeder installed on a document platform which feeds documents automatically onto the platform, and recovers document sheets which have been read. However if these sheets are not straight when they are transported onto the platform, the image formed on a recording medium will also be skewed and uncomfortable to view, hence some means was provided to correct the slant of the original document.
In conventional image forming apparatuses, any slant of a document sheet supplied by the automatic document feeder was first corrected by a width regulating plate of the document tray of the document feeder, and any slant of the document arising during transport onto the document platform was corrected by bringing the front edge of the sheet into contact with a document scale (specification plate) installed on the platform.
However, as the width regulating plate of the document tray of the automatic document feeder extends only up to the vicinity of a feed roller, a sheet which has a short length in contact with the width regulating plate, i.e. a sheet which has a short length in the transport direction relative to the width regulating plate, separates from the plate as soon as it is fed by the roller. Due to slight unevenness (non-uniformity) of the roller diameter, the sheet may therefore be transported on a slant (skew) relative to the transport direction.
When the transport method is such that the sheet transported onto the document platform temporarily stops in front of the correct reading position, and the front edge (or rear edge) is brought in contact with the document scale when the transport belt is restarted, the sheet may for example slip due to restarting of the belt so that it becomes slanted relative to the transport direction.
When positioning is performed by bringing the front edge of the sheet in contact with the document scale and the sheet is slanting relative to the transport direction when it comes in contact with the scale, it may shift to the left or right even if the slant is corrected. In the case of a thin paper document, the front edge of the paper may crumple or bend so that it is not correctly positioned.
Therefore when sheets of ordinary thickness are mixed with thin sheets, it may occur that the sheets of ordinary thickness are correctly positioned, whereas the thin sheets are not.
In the case of a double-sided document, after image processing of the first side is complete, the document is turned over by an inverting mechanism of the automatic document feeder, and is then transported again onto the platform to process the image on the second side. However if an inverting roller of the automatic document feeder inverting mechanism and the document platform are not parallel, the sheet will acquire a slant relative to the transport direction, and when its front edge comes in contact with the document scale, it will slip in an opposite sense to that in which it slipped when image processing of the first side was performed. This makes it difficult to adjust the transport mechanism and inversion mechanism of the automatic document feeder.
Moreover in the case of step feed in which a plurality of (e.g. 2) document sheets are continuously fed onto the document platform, document feed is temporarily stopped on the platform before the front edge of the (leading) sheet comes in contact with the scale, then after feed of the next sheet is complete, transport recommences so that the front edge of the leading sheet comes in contact with the scale. Hence in this case also, the sheet may become skewed relative to the transport direction due for example to slip when the transport belt starts again.
A method has been proposed in which positioning does not depend on a width regulating plate or document scale, the slant of the image being determined and thus corrected by electrical processing of a read image signal. However as electrical processing of the signal was complex and required a considerable time, it was inefficient for copying a plurality of sheets.
Further when a plurality of sheets is fed by an automatic document feeder, the adjustment of mechanical parts may change with time. This gradually leads to sheets becoming skewed relative to the transport direction and more paper jams during transport, which decreased the reliability of a sheet setting in a predetermined position.